Problem: In chemistry class, the girl to boy ratio is $9$ to $2$. If there are $18$ girls, how many boys are there?
Answer: A ratio of $9$ girls to $2$ boys means that there is a group of $9$ girls for every group of $2$ boys. If there are $18$ girls, then there are $2$ groups of $9$ girls each. Because we know the ratio is $9:2$ , there must be $2$ groups of $2$ boys each. There is a total of $4$ boys in chemistry class.